1. Field
The subject matter disclosed herein relates to wireless signal receivers, and more particularly to methods and apparatuses for use in detecting and filtering one or more undesired signal contributions that may interfere with the reception of one or more desired signals.
2. Information
Global navigation satellite systems (GNSS), such as the Global Positioning System (GPS), Galileo and the like, rely on an ability of a terrestrial navigation receiver to process a signal from a satellite position system (“SPS signal”) usually transmitted from transmitters fixed to space vehicles (SVs) to obtain pseudorange measurements from the navigation receiver to the transmitters. With pseudorange measurements to a sufficient number of transmitters and knowledge of locations of the transmitters, the navigation receiver may estimate its location.
An SPS signal may be encoded with a repeating sequential code. For example, SPS signals and other known communication signals may be transmitted and received using spread spectrum techniques (e.g., based on direct sequence spread spectrum (DSSS) modulation, direct-sequence code division multiple access (DS-CDMA) modulation, etc.). In one implementation, a receiver may attempt to determine a pseudorange measurement from a received SPS signal based, at least in part, on a detected code phase associated with the received SPS signal. Here, for example, such a receiver may detect such a code phase based upon a location of an energy peak detection within a code phase search window. However, given the weakness of SPS signals received from SVs at a long range, and the presence of multipath and noise, obtaining pseudorange measurements may be particular challenging at a mobile station such as a cellular phone given limited battery life and processing capability. Other communication signals and receivers may also exhibit similar characteristics or otherwise be affected in a multipath and/or noisy signaling environment.
In addition to overcoming multipath and noise, a receiver may also need to overcome undesired RF energy (e.g., a “jammer” signal) in or around the frequency band of an expected SPS signal (e.g., L1 at 1575.42 MHz (10.23 MHz×154) and L2 at 1227.60 MHz (10.23 MHz×120) for a GPS signal). Such undesired RF energy may be generated by other “transmitters” either intentionally or unintentionally. One such other transmitter may include any one of several sources of RF energy such as, for example, a transmitter on the mobile station for transmitting voice or data in a wireless communication network. With knowledge of characteristics of an undesired signal (e.g., carrier frequency, bandwidth, power level, etc.), a receiver may apply a notch filter to remove or substantially attenuate energy from an undesired signal in a received signal in processing an SPS signal and/or other communication signal.
In one implementation, a receiver at a mobile station may store or maintain a database of characteristics of different undesired signals that may be encountered in processing signals. Using such characteristics stored in a database, a receiver may program one or more notch filters to remove or attenuate the undesired signals. Information in such a database may be obtained using some “brute force” technique(s), e.g., by exhaustively profiling potential interfering signals in a wide area network (WAN) channel, and building a table of undesired signal carrier frequencies in a radio frequency driver of a navigation receiver. Disadvantageously, building such a database may be time consuming and/or may potentially inflate a driver code size beyond a tolerable level. Also, such a table may not be capable of addressing undesired signals from certain external sources and/or sporadically encountered/transmitted signals.